
A reference electrode is an
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
which has a stable and well-known
electrode potential
In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrod ...
. The high stability of the electrode potential is usually reached by employing a
redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
system with constant (buffered or saturated)
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number concentration'' ...
s of each participant of the redox reaction.
There are many ways reference electrodes are used. The simplest is when the reference electrode is used as a
half-cell to build an
electrochemical cell. This allows the
potential of the other half cell to be determined. An accurate and practical method to measure an electrode's potential in isolation (
absolute electrode potential) has yet to be developed.
Aqueous reference electrodes
Common reference electrodes and potential with respect to the standard hydrogen electrode (SHE):
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Standard hydrogen electrode (SHE) (E = 0.000 V) activity of H
+ = 1 Molar
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Normal hydrogen electrode (NHE) (E ≈ 0.000 V) concentration H
+ = 1 Molar
*
Reversible hydrogen electrode (RHE) (E = 0.000 V - 0.0591 × pH)
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Saturated calomel electrode (SCE) (E = +0.241 V saturated)
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Copper-copper(II) sulfate electrode (CSE) (E = +0.314 V)
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Silver chloride electrode (E = +0.197 V in saturated KCl)
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Silver chloride electrode (E = +0.210 V in 3.0 mol KCl/kg)
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Silver chloride electrode (E = +0.22249 V in 3.0 mol KCl/L)
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pH-electrode (in case of
pH buffered solutions, see
buffer solution
A buffer solution (more precisely, pH buffer or hydrogen ion buffer) is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or vice versa. Its pH changes very little when a small amount of strong acid or base is ...
)
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Palladium-hydrogen electrode
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Dynamic hydrogen electrode (DHE)
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Mercury-mercurous sulfate electrode (E = +0.64 V in sat'd K
2SO
4, E = +0.68 V in 0.5 M H
2SO
4) (MSE)
Nonaqueous reference electrodes
While it is convenient to compare between solvents to qualitatively compare systems, this is not quantitatively meaningful. Much as pK
a are related between solvents, but not the same, so is the case with E°. While the SHE might seem to be a reasonable reference for nonaqueous work as it turns out the platinum is rapidly poisoned by many solvents including acetonitrile causing uncontrolled drifts in potential. Both the SCE and saturated Ag/AgCl are aqueous electrodes based around saturated aqueous solution. While for short periods it may be possible to use such aqueous electrodes as references with nonaqueous solutions the long-term results are not trustworthy. Using aqueous electrodes introduces undefined, variable, and unmeasurable junction potentials to the cell in the form of a liquid-liquid junction as well as different ionic composition between the reference compartment and the rest of the cell. The best argument against using aqueous reference electrodes with nonaqueous systems, as mentioned earlier, is that potentials measured in different solvents are not directly comparable.
For instance, the potential for the Fc
0/+ couple is sensitive to solvent.
[Connelly, N. G., Geiger, W. E., "Chemical Redox Agents for Organometallic Chemistry", Chem. Rev. 1996, 96, 877.][Aranzaes, J. R., Daniel, M.-C., Astruc, D. "Metallocenes as references for the determination of redox potentials by cyclic voltammetry. Permethylated iron and cobalt sandwich complexes, inhibition by polyamine dendrimers, and the role of hydroxy-containing ferrocenes", Can. J. Chem., 2006, 84(2), 288-299. doi:10.1139/v05-262]
A quasi-reference electrode (QRE) avoids the issues mentioned above. A QRE with
ferrocene
Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, ...
or another
internal standard, such as
cobaltocene or
decamethylferrocene, referenced back to ferrocene is ideal for nonaqueous work. Since the early 1960s ferrocene has been gaining acceptance as the standard reference for nonaqueous work for a number of reasons, and in 1984, IUPAC recommended ferrocene (0/1+) as a standard redox couple. The preparation of the QRE electrode is simple, allowing for a fresh reference to be prepared with each set of experiments. Since QREs are made fresh, there is also no concern with improper storage or maintenance of the electrode. QREs are also more affordable than other reference electrodes.
To make a quasi-reference electrode (QRE):
# Insert a piece of silver wire into concentrated HCl then allow the wire to dry on a lint-free cleaning cloth. This forms an insoluble layer of AgCl on the surface of the electrode and gives you an Ag/AgCl wire. Repeat dipping every few months or if the QRE starts to drift.
# Obtain a
Vycor glass
frit (4 mm diameter) and glass tubing of similar diameter. Attach Vycor glass frit to the glass tubing with heat shrink Teflon tubing.
# Rinse then fill the clean glass tube with supporting electrolyte solution and insert Ag/AgCl wire.
# The
ferrocene
Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, ...
(0/1+) couple should lie around 400 mV versus this Ag/AgCl QRE in an acetonitrile solution. This potential will vary up to 200 mV with specific undefined conditions, thus adding an internal standard such as ferrocene at some point during the experiment is always necessary.
Pseudo reference electrodes
A pseudo reference electrode is a term that is not well defined and borders on having multiple meanings since ''pseudo'' and ''quasi'' are often used interchangeably. They are a class of electrodes named pseudo-reference electrodes because they do not maintain a constant potential but vary predictably with conditions. If the conditions are known, the potential can be calculated and the electrode can be used as a reference. Most electrodes work over a limited range of conditions, such as pH or temperature, outside of this range the electrodes behavior becomes unpredictable. The advantage of a pseudo-reference electrode is that the resulting variation is factored into the system allowing researchers to accurately study systems over a wide range of conditions.
Yttria-stabilized zirconia (
YSZ) membrane electrodes were developed with a variety of redox couples, e.g., Ni/NiO. Their potential depends on pH. When the pH value is known, these electrodes can be employed as a reference with notable applications at elevated temperatures.
[R.W. Bosch, D.Feron, and J.P. Celis, "Electrochemistry in Light Water Reactors", CRC Press, 2007.]
See also
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Auxiliary electrode The auxiliary electrode, often also called the counter electrode, is an electrode used in a three electrode electrochemical cell for voltammetric analysis or other reactions in which an electric current is expected to flow. The auxiliary electro ...
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Cyclic voltammetry
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Table of standard electrode potentials
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Working electrode
References
Further reading
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*{{cite journal, last1=O’Neil, first1=Glen D., last2=Buiculescu, first2=Raluca, last3=Kounaves, first3=Samuel P., last4=Chaniotakis, first4=Nikos A., title=Carbon-Nanofiber-Based Nanocomposite Membrane as a Highly Stable Solid-State Junction for Reference Electrodes, journal=Analytical Chemistry, volume=83, issue=14, year=2011, pages=5749–5753, issn=0003-2700, doi=10.1021/ac201072u, pmid=21662988, s2cid=14419383, url=https://semanticscholar.org/paper/f27548f96e6578ee4603887c01b6de42c06f0575
Electrodes